Non-volatile magnetic random access memories (MRAMs) have been proposed as candidates to replace conventional dynamic random access memories (DRAMs) and hard disk drives. Such memory devices make use of the giant magnetoresistance (GMR) and tunneling magnetoresistance (TMR) sensing techniques. Currently implemented MRAM devices utilize two magnetic layers magnetized in-plane or out-of-plane so that variations in the GMR and/or TMR may be measured. The magnetic layers have magnetic orientations which are either parallel or anti-parallel to each other, which create four possible magnetic states and give rise to two different GMR or TMR resistances associated with bits “0” and “1”.
A different memory cell structure makes use of the extraordinary Hall effect in ferromagnetic materials. A memory cell has a ferromagnetic layer possessing perpendicular magnetic anisotropy with magnetic moment oriented perpendicular to the plane of the ferromagnetic layer. The extraordinary Hall resistance is exhibited between first and second ends of the ferromagnetic layer across a path which intersects a bias current path between third and fourth ends of the ferromagnetic structure. Such a magnetic layer has two stable magnetic orientations: up and down, and gives rise to two different extraordinary Hall values +RH and −RH associated with bits “0” and “1”. A magnetic memory unit of this type with enhanced values of RH has been disclosed in U.S. Pat. No. 7,463,447 B2 (2008) to A. Gerber, whose disclosure is incorporated herein by reference.
Another memory cell structure makes use of the normal, or ordinary, Hall effect in low carrier density materials such as semiconductors. A memory cell has a low carrier density material film in the shape of a cross which constitutes a Hall sensor, on top of which a spacer is placed to prevent electric current leakage, and on top of that a ferromagnetic dot, or ferromagnetic dots are deposited. Such magnetic dots have two stable magnetic orientations up and down. Ferromagnetic dots are placed at locations that induce strong magnetic stray flux through the sensor. The normal Hall resistance in the low carrier density film is sensitive to cumulative stray flux from the magnetized ferromagnetic dots, and gives rise to two different normal Hall values +RH and −RH associated with bits “0” and “1”. A magnetic memory unit of this type has been disclosed in U.S. Patent Application Publication 2008/0205129 to J. Stephenson, B. Shipley, and D. Carothers, whose disclosure is incorporated herein by reference.
In an effort to further increase the ultimate storage density of MRAMs, several multi-state structures and storage schemes have been proposed using both in-plane and perpendicular anisotropy materials. Angular dependent four-state tunneling magnetoresistance cells were proposed by Uemura et al., in “Four-State Magnetoresistance in Epitaxial CoFe-Based Magnetic Tunnel Junctions,” IEEE Transactions on Magnetics, volume 43, pages 2791-2793 (2007), which is incorporated herein by reference.
Four-state dual spin valve GMR storage was proposed by Law et al., in “Magnetoresistance and Switching Properties of Co—Fe/Pd-Based Perpendicular Anisotropy Single- and Dual-Spin Valves,” IEEE Transactions on Magnetics, volume 44, pages 2612-2615 (2008), which is incorporated herein by reference. A four-state single-layer Fe film device was proposed by Yoo et al., in “Four Discrete Hall Resistance States in Single-Layer Fe Film for Quaternary Memory Devices,” Applied Physics Letters, volume 95, 202505 (2009), which is incorporated herein by reference.
A memory cell having two separated ferromagnetic layers was disclosed in U.S. Pat. No. 7,379,321 to D. Ravelosona and B. D. Terris, whose disclosure is incorporated herein by reference. U.S. Pat. No. 5,361,226, to Taguchi, et al., whose disclosure is incorporated herein by reference, describes a magnetic thin film memory device having information recorded in a magnetic thin film by the direction of magnetization. The disclosure states that the film is adapted to reproduce the recorded information on the basis of the voltage generated as a result of the change of the magnetization direction due to the extraordinary Hall effect.
U.S. Pat. No. 6,727,537, to Wunderlich, whose disclosure is incorporated herein by reference, describes a magnetic memory device based on easy domain wall propagation and the extraordinary Hall effect, and which is stated to include a “perpendicular-to-plane” magnetic electrically conductive element.